rpm spec: fix version numbering when building alpha, beta, rc

We want to ensure smooth upgrades when doing rpm -U throughout the
release cycle so this change implements the scheme documented at:
http://fedoraproject.org/wiki/Packaging%3aNamingGuidelines#NonNumericRelease

Signed-off-by: Dwight Engen <dwight.engen@oracle.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>